Muscle strength and qualitative jump-landing differences in male ...

Beutler et al. 665Figure 1. Jump-Landing Task: Participants jumped from a 30cm high box onto a forceplate and then immediatelyrebounded back up in a maximal jump.the required horizontal distance or did not vertically jumpafter the initial landing, that trial was discarded and thejump-landing manoeuvre repeated.Two tripod-mounted digital camcorders recordeda frontal and sagittal view of each jump, at a distance of16 and 13 feet, respectively. Cameras were levelled usingthe built-in level on each tripod. All jump-landing videoswere analyzed at a later time by trained raters using theLanding Error Scoring System (LESS) (Boling et al.,2005; DiStefano et al., 2009; Padua et al., 2009). TheLESS is a clinical assessment tool that reliably identifiesindividuals with potentially high-risk biomechanics(Padua et al., 2009). Jump-landing quality is assessed byanalyzing videotapes of the jump-landing task in the sagittaland frontal planes. Scoring is based on the presenceor absence of specific landing characteristics as describedin the Appendix. Individually scored items are totalled tocreate an overall LESS score (range 0-17), with higherscores (more errors) indicative of higher-risk landingtechnique. Overall LESS scores were averaged over threevalid jump trials for analyses.Data analysisIndependent t-tests were used to compare gender meandifferences for subject anthropometrics, strength values,and overall LESS score. Separate univariate and multivariatelinear regressions were used to determine thepredictive validity of subjects’ anthropometrics and musclestrength using overall LESS score as the dependentvariable. Individual models for males and females werefit so as to assess gender-specific relationships.Additionally, factor analysis was performed on individualLESS items to identify inter-related movementerrors [For factor analysis, a positive score on individual LESSitems was defined as an Error if was judged to occur on at least2 of 3 trials (items 1-15)]. We used varimax rotation anditerated analyses. Two “general purpose” items on theLESS (#16 and #17) were excluded from analysis becauseof possible collinearity with the other 15 items. Regressiondiagnostics indicated no collinearity in the remainingLESS items (largest condition index of 4.6). Pooled variancet-tests were used to compare factor means betweengenders as well as factor frequency by gender. The significancelevel for all analyses was set a priori at α = 0.05.ResultsLanding technique: the Landing Error Scoring System(LESS)Total overall Landing Error Scoring System (LESS)scores were significantly different (p < 0.001) for malesand females. Males’ mean score was 4.65 ± 1.69 (Range0.00-10.67). The mean score for females was 5.34 ±1.51(range 0.33-11.00).Factor analysis is a statistical method used to detectstructure in the relationships between variables, or inother words, identify patterns among the observed variables.Using data from individual LESS items, factoranalyses revealed five groups of related errors (orthogonalfactors): Factor 1 - decreased sagittal trunk, hip and kneeflexion at initial ground contact (LESS Items L3, L2,andL1); Factor 2 - valgus knee and feet wide at initial contact(Items L5 ,L15, and L7); Factor 3 - toes out and kneesflexed at initial contact (Items L10 and L1); Factor 4 -heelstrike landing and asymmetric footstrike landing;(Items L4 and L11) and Factor 5 - less sagittal flexionover the landing phase (Items L12, L13, and L14). Thesefive factors all had Eigenvalues greater than one, andcollectively accounted for 67% of the covariance betweenthe 15 LESS items.Pooled variance t-tests between genders on these 5 factorsshowed that females were significantly more likely thanmales to land with: less hip and knee flexion at initialcontact (Factor 1, p < 0.001); more knee valgus and widerlanding stance (Factor 2, p < 0.001); and less flexiondisplacement over the entire landing phase (Factor5, p

666Muscle strength and movement pattern differencesTable 2. Number of subjects scoring an error for each factor 1,2 .Male(N=1707)Female(N=1046)Factor N % N %Factor 1 Poor Sagittal Flexion – Stance 559 32.8% 448 42.6%Factor 2 Valgus Knee & Feet Too Wide 428 25.1% 386 36.7%Factor 3 Toes Out & Knees Flexed 503 29.5% 154 14.6%Factor 4 Heelstrike & Uneven Footstrike 751 44.1% 273 26.0%Factor 5 Poor Sagittal Flexion – Landing 582 34.1% 472 44.9%1 Factors generated from factor analysis loading. 2 For items 1-15, a positive score was defined as an Error onat least 2 of 3 trials. For items 16 & 17, a positive score was defined as Average on at least 2 of 3 trials orPoor/ Stiff on at least 1 of 3 trials.showed that males and females were significantly differentfor all investigated anthropometric (height, weight,BMI, navicular drop, and Q-angle) and strength variables(Tables 3 and 4).Table 3. Subject biometrics. Data are means (±SD).Males(n = 1,607)Females(n = 994)Height (m) 1.78 (.07) 1.66 (.07) *Weight (kg) 77.4 (12.3) 63.0 (7.8) *Body Mass Index 24.3 (3.1) 22.9 (2.3) *Navicular Drop (mm) 7.5 (2.8) 7.0 (2.6) *Q-Angle (º) 8.6 (4.5) 11.6 (4.9) ** p < 0.001Table 4. Muscle strength normalized to body mass (N·kg -1 ).Data are means (±SD).Males(n = 1,607)Females(n = 994)Quadriceps .49 (.09) .41 (.09) *Hamstrings .24 (.05) .21 (.05) *Hip external rotation .21 (.04) .17 (.03) *Hip internal rotation .19 (.04) .18 (.04) *Gluteus maximus .26 (.07) .23 (.07) *Gluteus minimus .34 (.08) .30 (.07) ** p < 0.001Predicting poor landingDue to missing data values, univariate and multivariateregression models were analyzed for 2,734 participants(females = 1046; males = 1688). Univariate analysis formales indicated that low BMI, increased Q-angle, andpoor gluteus medius strength were individually predictiveof poor landing technique (higher LESS score). Howeveras a group, multivariate modelling showed extremelylimited predictive value (combined R 2 =.016; p < 0.01)and only lower BMI and weak hip internal rotationstrength significantly contributed to poorer landing technique(Table 5). For females, univariate analyses suggestedthat weaker hamstrings and weaker gluteus mediusstrength were important predictors of poorer jump-landing(Table 6). However, in contrast to males, we were notable to strongly predict specific contributors to landingerror in females with multivariate analysis (p = 0.098).Again, the predictive value of all these variables as agroup was very limited (combined R 2 = 0.014).DiscussionMovement pattern differences can be assessed by theLESSThe ability to quickly and reliably identify individuals athigh risk of injury is critical to injury prevention efforts.Because high-risk movement patterns have been linked toACL injury risk (Hewett et al., 2005; Krosshaug et al.,2007), the ability to rapidly screen for these movementpatterns could facilitate the implementation of large-scaleinjury prevention efforts. Our results indicate that a qualitativemovement screen, the Landing Error Scoring System(LESS) can accurately characterize the jump-landingcharacteristics of a large cohort of males and females.Females demonstrated poorer overall landing technique(5.34 ± 1.51 vs. 4.65 ± 1.69 for males) and showed lessknee flexion, less hip flexion and more valgus collapse.These results are similar to studies of common movementtasks using traditional laboratory biomechanics (Chappellet al., 2002; Hewett et al., 2006; Malinzak et al., 2001;Padua et al., 2009). Previous work has shown that LESSscore is correlated with high-risk movement patterns asmeasured by traditional motion analysis (Padua et al.,2009). Taken together these results suggest that the LESSaccurately characterizes jump-landing movements. Theresults of the present investigation do not allow us todetermine which differences are specifically correlatedTable 5. Univariate and multivariate regression results – Males.Univariate 1 Multivariate 1Variable β SE t-value p-value β SE t-value p-valueBMI -.091 .321 -3.77

Beutler et al. 667Table 6. Univariate and multivariate regression results – Females.Univariate 1 Multivariate 1Variable β SE t-value p-value β SE t-value p-valueBMI -.024 .020 -.77 .44 -.040 .022 -1.22 .22Q-Angle .026 .010 .85 .40 .020 .010 .64 .52Navicular Drop -.017 .018 -.57 .57 -.020 .018 -.65 .52Quadriceps -.011 .554 -.36 .72 .045 .722 1.11 .29Hip Ext. Rotators -.023 1.426 -.73 .47 .041 1.931 .97 .33Hip Int. Rotators -.052 1.264 -1.70 .09 -.026 1.822 -.58 .57Hamstrings -.067 1.008 -2.17 .03 * -.070 1.387 -1.65 .10Gluteus Maximus -.005 .717 -.18 .86 .031 .786 .91 .37Gluteus Medius -.081 .646 -2.6 .01 -.094 .847 -2.32 .02 ** Statistically significant. 1 Univariate betas are not adjusted for any other variable; multivariate betas are adjusted forany other variables listed in the table.with an increased risk of injury. The error of measurementfor the LESS score is greater than observed gender differences,suggesting that individual variability may limit thepredictive capacity of the LESS. However, the ability ofthe LESS to predict injury in the military academy populationis an area of ongoing study.Implications for injury risk based on Landing ErrorPatternsWhile singular aspects of jump-landing technique contributeto high-risk landings, it is possible that a combinationof multiple factors is more predictive of overall risk.We were able to determine common landing-error patternsfor females and males using factor analysis. Strikingly,each error pattern detected has been suggested inprevious literature to contribute to ACL injury: 1) decreasedsagittal flexion at initial ground contact (Chappellet al., 2007); 2) valgus knee (Ford et al., 2003; Hewett etal., 1999; 2005) and feet wide at initial contact; 3) toesout and knees flexed at initial contact (DiStefano et al.,2009); 4) heelstrike landing and asymmetric footstrikelanding (Boden et al., 2009); and 5) lack of sagittal flexionover the landing (Chappell et al., 2007). While bothmale and female cadets exhibited these combinations ofhigh-risk movement, clear gender tendencies towardsdifferent patterns were detected (Table 2). Similar toprevious reports, 42.6% of females in our cohort exhibitedshallow sagittal flexion angles (trunk, hip, and knee) atground contact (Factor 1) and 44.9% demonstrated poorknee flexion displacement over the entire landing phase(Factor 5). These flexion faults and propensity towardsvalgus during landing are consistent with many priorinvestigations (Chappell et al., 2002; 2007; Colby et al.,2000; Malinzak et al., 2001). In contrast to these findings,a recent study reported females had increased knee flexionat the time of ACL injury versus males (Krosshaug etal., 2007). However, knee and hip flexion may be taskdependent,which may explain the differing results.We also found that females were more likely thanmales to land with an excessively wide stance in combinationwith knee valgus (Factor 2). This has not beenpreviously reported in traditional biomechanical investigations.This wider landing stance may represent an adaptiveprecursor to subsequent valgus collapse, or it maysimply reflect a wider average pelvic width in femalecadets. We are directly measuring pelvic width in anongoing study. Analysis of that data may allow us tobetter describe the phenomenon of wide landing stance infemales.Males exhibited different landing-error patternsthan females. The most common male factor was heelstrikelanding/uneven footstrike during landing (Factor 4),which occurred in 44.1% of our male population. Maleswere also more likely than females to land with “toes-out”(tibial external rotation) (Factor 3). These male errorpatterns have only recently been reported (Boden et al.,2009; Krosshaug et al., 2007) and are less familiar, perhapsless understood than female themes. While cadavericand biomechanical modelling studies suggest that ACLstrain is greater with knee internal rotation, analyses ofactual injury mechanism describe an external rotatoryforce on the knee, suggesting this position is indeed highrisk(Shimokochi and Shultz, 2008). The effects ofasymmetric foot strike and a heels-first landing are lessdiscussed in ACL injury literature. One possible implicationcomes from preliminary work by Boden and colleagueswho propose that a flat-foot landing reduces theability of the calf muscles to dampen the ground reactionforces before they reach the knee (Shimokochi andShultz, 2008).Predicting poor landingIn addition to classifying common patterns of movement,we attempted to identify relationships between measuredanthropometric and strength elements and observed landingmovement patterns. In the univariate analysis of individualvariables, our results suggest that low BMI contributesmore to jump-landing movements in male cadetsthan in female cadets. The relationship between this findingand previous reports of higher BMI being associatedwith increased injury in military and other populations isunclear (Jones et al., 1986; Knapik et al., 1991; Uhorchaket al., 2003). One possible explanation is that fatiguecould preferentially worsen movement patterns in thosewith high BMI. Future studies comparing movementpatterns under fatigued and non-fatigued conditionswould facilitate further analysis. We also found that hiprotator strength exerts minimal influence on poor landingtechnique in either gender. This appears to contrast withrecent thinking in injury prevention and rehabilitationwhere strengthening hip rotators is thought to reduceinjury susceptibility (Hewett et al., 1999; Mandelbaum etal., 2005). Our pending analysis of traditional biomechanicalmeasures during the jump-landing task may helpexplain these apparently contradictory results.

668Muscle strength and movement pattern differencesMost importantly, our results clearly demonstratethat muscle strength and anthropometric factors do NOTcontribute significantly to landing movement patterns asmeasured by the LESS. Multivariate analysis showed verylittle contribution from anthropometric and strength valuesto overall movement patterns (R 2 = 0.016 for males,0.014 for females). This is a novel finding. The primarytenants of many ACL injury prevention programs averthat increased lower extremity muscular will translate toimproved movement patterns during athletic manoeuvres.In contrast, our findings suggest that simply changing anathlete’s alignment, BMI, or muscle strength may notultimately improve his or her movement patterns. Rather,landing movements seem to be primarily determined bycharacteristics other than strength, alignment, or bodymass. Future investigations should verify this lack ofcorrelation using traditional biomechanical methods. Ifconfirmed, the absence of a correlation in anthropometricsand muscle strength with an individual’s movement patternwould necessitate a paradigm shift in future injuryprevention intervention design.Study limitationsWe caution that the use of a military population may limitthe generalizability of these results to all populations ofyoung athletes. Specifically, the overall athleticism, BMI,or fitness of the military academy population may besignificantly different than that of other populations. Additionally,although we have measured landing movementpatterns in terms of “errors”—a common practice inqualitative movement screening—we recognize that differencesin movement patterns cannot definitively beclassified as errant unless prospectively linked to injuryrisk.ConclusionMale and female military cadets have differences in jumplandingtechnique as assessed through a qualitativemovement screen. Females demonstrate distinct landingmovement patterns versus males. Landing-error patternsmore common in males and those more common in femalescontain features that have been previously postulatedto increase ACL injury risk. Most importantly, BMI,navicular drop, Q-angle, and muscular strength do notsignificantly predict movement patterns in either male orfemale cadets. We are collecting ACL injury incidencedata from this cohort over their 4-year academy careers.This injury data may allow us to link prospective, modifiablerisk factors with LESS scores, and ultimately withthe risk of subsequent ACL injury.DisclosureThe views expressed here are those of the authors and do not representthe policy of the United States Air Force or Department of Defense.ReferencesArendt, E. and Dick, R. (1995) Knee injury patterns among men andwomen in collegiate basketball and soccer. NCAA data andreview of literature. American Journal of Sports Medicine 23,694-701.Arendt, E.A., Agel, J. and Dick, R. 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Beutler et al. 669Mechanisms of anterior cruciate ligament injury in basketball:video analysis of 39 cases. American Journal of SportsMedicine 35, 359-367.Lephart, S.M., Ferris, C.M., Riemann, B.L., Myers, J.B. and Fu, F.H.(2002) Gender differences in strength and lower extremitykinematics during landing. Clinical Orthopaedics & RelatedResearch 401, 162-169.Malinzak, R.A., Colby, S.M., Kirkendall, D.T., Yu, B. and Garrett, W.E.(2001) A comparison of knee joint motion patterns betweenmen and women in selected athletic tasks. ClinicalBiomechanics 16, 438-445.Mandelbaum, B.R., Silvers, H.J., Watanabe, D.S., Knarr, J.F., Thomas,S.D., Griffin, L.Y., Kirkendall, D.T. and Garrett, W., Jr. (2005)Effectiveness of a neuromuscular and proprioceptive trainingprogram in preventing anterior cruciate ligament injuries infemale athletes: 2-year follow-up. American Journal of SportsMedicine 33, 1003-1010.Myer, G.D., Ford, K.R., Barber Foss, K.D., Liu, C., Nick, T.G. andHewett, T.E. (2009) The relationship of hamstrings andquadriceps strength to anterior cruciate ligament injury infemale athletes. Clinical Journal of Sports Medicine 19, 3-8.Onate, J.A., Guskiewicz, K.M., Marshall, S.W., Giuliani, C., Yu, B. andGarrett, W., Jr. (2005) Instruction of jump-landing techniqueusing videotape feedback: altering lower extremity motionpatterns. American Journal of Sports Medicine 33, 831-842.Padua, D.A., Marshall, S.W., Boling, M.C., Thigpen, C.A., Garrett,W.E., Jr. and Beutler, A.I. (2009) The landing error scoringsystem (less) is a valid and reliable clinical assessment tool ofjump-landing biomechanics: the JUMP-ACL study. AmericanJournal of Sports Medicine 37, 1996-2002.Shimokochi, Y. and Shultz, S.J. (2008) Mechanisms of noncontactanterior cruciate ligament injury. Journal of Athletic Training43, 396-408.Uhorchak, J.M., Scoville, C.R., Williams, G.N., Arciero, R.A., St.Pierre, P. and Taylor, D.C. (2003) Risk factors associated withnoncontact injury of the anterior cruciate ligament: aprospective four-year evaluation of 859 West Point cadets.American Journal of Sports Medicine 31, 831-842.Woodland, L. and Francis, R. (1992) Parameters and comparisons of thequadriceps angle of college-aged men and women in the supineand standing positions. American Journal of Sports Medicine20, 208-211.Key points• Important differences in male and female landingtechnique can be captured using a qualitativemovement screen: the Landing Error Scoring System(LESS)• Female cadets were more likely to land with shallowsagittal flexion, wide stance width, and more pronouncedknee flexion.• Male cadets were more likely to exhibit a heel-strikeor asymmetric foot-strike and to land with toe out.• Lower extremity muscle strength, Q-angle, andnavicular drop do not significantly predict landingmovement pattern in male or female cadets.AUTHORS BIOGRAPHYAnthony I. BEUTLEREmploymentMajor, US Air Force,Chief, Injury PreventionResearch Lab, Uniformed ServicesUniversityDegreeMDResearch interestsACL Injury, Injury Prevention, MusculoskeletalEducationE-mail: abeutler@usuhs.milSarah J. de la MOTTEEmploymentPost Doctoral Fellow, Injury PreventionResearch Lab, Uniformed Services UniversityDegreePhD, ATCResearch interestsACL Injury, Ankle Injury, Injury PreventionE-mail: sarah.delamotte@usuhs.milStephen W. MARSHALLEmploymentAssociate Professor, Department of Epidemiology,School of Public HealthUniversity of North Carolina, Chapel HillDegreePhDResearch interestsInjury Prevention, Violence PreventionE-mail: smarshall@unc.eduDarin A. PADUAEmploymentAssociate Professor, Department of Exercise& Sport Science, University of NorthCarolina, Chapel HillDegreePhD, ATCResearch interestsACL Injury Prevention, NeuromuscularControlE-mail: dpadua@email.unc.eduBarry P. BODENEmploymentBoard Certified Orthopaedic SurgeonThe Orthopaedic Center, Rockville, MDDegreeMDResearch interestsACL Injury, Head InjuryE-mail: bboden@starpower.net Anthony I. BeutlerUniformed Services University of the Health Sciences, Bethesda,MD, USA

670Muscle strength and movement pattern differencesLESS Item1 Knee flexion angleat initial contact2 Hip flexion angle atinitial contact3 Trunk flexion angleat initial contact4 Ankle plantarflexionangle atinitial contact5 Knee valgus angleat initial contact6 Lateral trunk flexionangle at initialcontact7 Stance width –Wide8 Stance width –Narrow9 Foot position – ToeIn10 Foot position – ToeOut11 Symmetric initialfoot contact12 Knee flexion displacement13 Hip flexion at maxknee flexion14 Trunk flexion atmax knee flexion15 Knee valgus displacementAppendixLESS Item ScoringOperational DefinitionAt the time point of initial contact, if the knee of the test legis flexed more than 30 degrees, score YES. If the knee is notflexed more than 30 degrees, score NO.At the time point of initial contact, if the thigh of the test legis in line with the trunk then the hips are not flexed and scoreNO. If the thigh of the test leg is flexed on the trunk, scoreYES.At the time point of initial contact, if the trunk is vertical orextended on the hips, score NO. If the trunk is flexed on thehips, score YES.If the foot of the test leg lands toe to heel, score YES. If thefoot of the test leg lands heel to toe or with a flat foot, scoreNO.At the time point of initial contact, draw a line straight downfrom the center of the patella. If the line goes through themidfoot, score NO. If the line is medial to the midfoot, scoreYES.At the time point of initial contact, if the midline of the trunkis flexed to the left or the right side of the body, score YES.If the trunk is not flexed to the left or right side of the body,score NO.Once the entire foot is in contact with the ground, draw a linedown from the tip of the shoulders. If the line on the side ofthe test leg is inside the foot of the test leg then score greaterthan should width (wide), and score YES. If the test foot isinternally or externally rotated, grade the stance width basedon heel placement.Once the entire foot is in contact with the ground, draw a linedown from the tip of the shoulders. If the line on the side ofthe test leg is outside of the foot then score less than shoulderwidth (narrow), score YES. If the test foot is internally orexternally rotated, grade the stance width based on heelplacement.If the foot of the test leg is internally rotated more than 30degrees between the time period of initial contact and maxknee flexion, then score YES. If the foot is not internallyrotated more than 30 degrees between the time period ofinitial contact to max knee flexion, score NO.If the foot of the test leg is externally rotated more than 30degrees between the time period of initial contact and maxknee flexion, then score YES. If the foot is not externallyrotated more than 30 degrees between the time period ofinitial contact to max knee flexion, score NO.If one foot lands before the other or if one foot lands heel totoe and the other lands toe to heel, score NO. If the feet landsymmetrically, score YES.If the knee of the test leg flexes 45 degrees more than theangle at the position of initial contact to max knee flexion,score YES. If the knee of the test leg does not flex morethan 45 degrees, score NO.If the thigh of the test leg flexes more on the trunk frominitial contact to max knee flexion angle, score YES. If thethigh does not flex more on the trunk, score NO.If the trunk flexes more from the point of initial contact tomax knee flexion, score YES. If the trunk does not flexmore, score NO.At the point of max knee valgus on the test leg, draw a linestraight down from the center of the patella. If the line runsthrough the great toe or is medial to the great toe, score YES.If the line is lateral to the great toe, score NO.CameraViewErrorConditionLESSScoreSide No Y=0N=1Side No Y=0N=1Side No Y=0N=1Side No Y=0N=1Front Yes Y=1N=0Front Yes Y=1N=0Front Yes Y=1N=0Front Yes Y=1N=0Front Yes Y=1N=0Front Yes Y=1N=0Front No Y=0N=1Side No Y=0N=1Side No Y=0N=1Side No Y=0N=1Front Yes Y=1N=0

Beutler et al. 67116 Joint displacement Watch the sagittal plan motion at the hips and knees frominitial contact to max knee flexion angle. If the subject goesthrough large displacement of the trunk, hips, and knees thenscore SOFT. If the subject goes through some trunk, hip,and knee displacement, but not a large amount, score AV-ERAGE. If the subject goes through very little, if any trunk,hip, and knee displacement, score STIFF.17 Overall impression Score EXCELLENT if the subject displays a soft landingand no frontal plane motion at the knee. Score POOR if thesubject displays a stiff landing and large frontal plane motionat the knee. All other landings, score AVERAGE.SideSide,FrontAverageor Stiff(doublepenaltyfor Stiff)Averageor Poor(doublepenaltyfor Poor)Soft=0Avg=1Stiff=2Ex=0Avg=1Poor=2